Schematic of the rat OPRM1 gene structure and alternative splicing. A. Genomic structure of the rat OPRM1gene. Exons and introns are showed by boxes and horizontal lines, respectively. Translational start and termination sites are indicated by downward and upward lines on exon boxes, respectively. Exons are numbered based upon their time of discovery, as previously reported. B. Alternatively spliced variants of the rat OPRM1 gene. Exon composition for each alternatively spliced variant was indicated by appropriate exon boxes. The lines between exons are introns that are spliced out during splicing. Translation start and stop points are shown by bars below and above exon boxes, respectively.